1. Field of the Invention
The present invention relates to a power transmission device, and to a portable power working machine such as a concrete cutter, which is provided with such a power transmission device. In particular, the present invention relates to a power transmission device configured to perform the power transmission by means of a power transmission member such as an endless belt or an endless chain wound around a pair of transmission wheels such as pulleys or sprocket wheels to thereby interconnect the transmission wheels. The present invention also relates to a portable power working machine such as a concrete cutter, which is provided with the above-mentioned power transmission device.
2. Description of the Related Art
In the case of a portable power working machine such as a concrete cutter, a winding transmission type power working machine has been widely employed as a power transmission device for transmitting the power of a prime mover (internal combustion engine or electric motor) to a working member such as a cutter, wherein the power is transmitted by means of a power transmission member (such as a V-belt) which is disposed between and wound around a pair of transmission wheels (such as V-pulleys).
In the power transmission device of this type, the belt is required to be kept under a predetermined tension in order to minimize any loss in the transmission of motive power. For the purpose of meeting such a requirement, a belt structure with a tension roller has been proposed. However, in the case of portable power working machines where it is strongly desired to miniaturize and lighten the machine as much as possible, it may not be possible to install such a tension roller due to the limited space available in the machine. Further, since the initially set tension of the belt generally would gradually loosen due to the elongation or wear of the belt, depending on the duration or environments of use, the employment of only the aforementioned tension roller may not be sufficient in many cases.
With a view toward solving this problem, it has also been proposed to make the distance between the axes of a pair of pulleys (the distance of separation between rotational axes) adjustable to thereby enable the belt to be kept under a constant predetermined tension. As for the method of adjusting the distance between the axes of a pair of pulleys, movement of a supporting member supporting one of the pulleys in a direction orthogonal to the rotational axis of the pulley by way of so-called screw feeding using a threaded rod (bolt and the like) has been used in the past.
However, the aforementioned power transmission device where the distance between the axes of a pair of pulleys is made adjustable by way of the screw feeding using a threaded rod is problematic when exchanging the belt or adjusting the distance between the axes (the adjustment of the tension of the belt). Specifically, it takes a lot of time and trouble to loosen the number of bolts employed for fixing the supporting member as well as to suitably clamp or loosen the aforementioned threaded rod, etc. In particular, in a situation where a V-belt having a relatively large thickness is employed, one of the pulleys (or the supporting member) must be moved a fairly large amount (generally, 10 mm or so in distance) when exchanging the belt. In this example, according to the aforementioned screw feeding, even if the threaded rod is turned one revolution, the pulley can be moved by a small distance corresponding to only one pitch of the screw (1 mm or so, in general), so that the threaded rod must be rotated many revolutions when exchanging the used belt with a new belt. Moreover, the rotation of the threaded rod requires employment of tools such as a spanner or wrench. These tools can rotate only 90° at most in each rotating manipulation thereof as the further rotation of these tools is obstructed by the presence of, among other things, the aforementioned supporting member. Due to the limited rotating angle of these tools as mentioned above, these tools must be usually removed from the threaded rod before finishing the needed rotation of the threaded rod, thus requiring time-consuming manipulations of tools when exchanging the belt as well as taking a lot of time and trouble in the adjustment of the distance between the pair of axes as mentioned above.
Additionally, since the belt is mounted under a great amount of tension, a very large force is needed to rotate the threaded rod when making the aforementioned adjustment of the distance between the axes. At the same time, because the determination as to whether the degree of tension of the belt is appropriate is based largely on the operator's perception, the tension of the belt is liable to be either insufficient or excessive, thus making the operation of adjusting the distance between a pair of the axes difficult or troublesome.